The devices used in such amplifiers, which may be semiconductor devices such as transistors, or therminoic tubes, may be required to handle large radio frequency (RF) currents which may fluctuate, for example with the degree of modulation of the RF signal being amplified.
It is an unfortuante feature of such devices that their internal temperature will vary as a function of the average RF power handled, due to the heating effect of the RF current. The resulting changes of internal temperature produce corresponding changes in the condustion point of the device and therefore related changes of gain.
For example, in a silicon transistor, the base-emitter voltage necessary to initiate collector current flow, Vbe, changes by 2.1 mV for each degree centigrade change in the temperature of the silicon structure of the device.
Radio frequency amplifiers in television transmitters are commonly run in Class AB mode, in which a degree of non-linearity in the power-in/power-out characteristic of the amplifier is tolerated at high signal levels. However to achieve optimum performance from such Class AB amplifiers, the biassing level of the amplifying device is critical. It will be appreciated that variation in the conduction point of the device, resulting from internal temperature changes caused by radio frequency Dower level variation can give rise to unwanted and varying distortion of the output signal from the device, as the average radio frequency power level varies.
In a radio frequency amplifier within television transmitting apparatus the problem is particularly severe as the amplitude of the modulated radio frequency signal and hence the average radio frequency power handled by the device can undergo rapid swings as the signal varies from levels corresponding to white (minimum power) to black (maximum power). The effect of resulting non-linearities on the transmitted vision signal due to thermal changes within the device can be most noticeable.
The thermal changes associated with the internal heating of the device are rapid, and occur in the internal structure of the device. It is therefore not possible to monitor and correct for such changes by means such as an externally attached temperature sensor forming part of a compensation arrangement, such as may be used with radio frequency power amplifying devices to correct for relatively long-term thermal changes due to ambient operating conditions.
It is thus an object of the present invention to obviate at least in part the deleterious effect of such rapid internal temperature changes due to variations in radio frequency power level, on the performance of devices employed in radio frequency amplifiers.